One common time division duplex system configuration employs equipment located at a centralized location and equipment that is remotely located from the centralized location. The equipment located at a centralized location is controlled by a wireless service provider. The equipment that is located at the centralized locations consists of at least one base station and can also include distributed antenna “host” or “hub” units. The remotely located equipment consists of subscriber units or subscriber stations, which are typically not controlled by the wireless service provider. Optional equipment at other remote locations (which may or may not be controlled by the wireless service provider) consists of remote antenna units or remote radio heads.
One such system is a time division duplex system in which radio frequency (RF) signals are communicated between a base station and one or more subscriber stations. In one system configuration, the subscriber station is communicatively coupled to the base station by an antenna on the base station. In another system configuration, the subscriber station is communicatively coupled to the base station by an optical fiber and a remote antenna. In yet another system configuration, subscriber stations are communicatively coupled to the base station by at least one antenna on the base station and by one or more optical fibers and associated remote antennas. A distributed antenna system (DAS) is a system that includes optical fiber links to communicatively couple the base station to remote antenna units, which are communicatively coupled to subscriber stations located within the broadcast range of the remote antenna units.
In any of these configurations, time division duplex systems use the same channel band for downlink and uplink transmissions. Although it is theoretically possible to separate simultaneous downlink and uplink transmissions using directional couplers, the non-ideal nature of directionality in real couplers and the enormous differential between transmit and receive power levels will cause downlink/uplink interference if the transmissions are simultaneous. As a result, the base station or subscriber station cannot transmit and receive at the same time. The base station in the time division duplex system must either be transmitting in the downlink or receiving in the uplink but not both simultaneously. Likewise, the subscriber station must be receiving in the downlink or transmitting in the uplink but not both simultaneously.
The time division duplex system transmits time division duplex frames having an uplink frame (subscriber station to base station) and a downlink frame (base station to subscriber station). The time division duplex systems have a distance limitation between the base station and the subscriber station. The distance limitation is based on an allowed turn-around time that is provided between the downlink frames and the uplink frames of the time division duplex frame. If the round-trip propagation delay of the signals traveling the communication link is greater than the difference between the base station turn-around time and the minimum allowed subscriber station turn-around time, the base station and/or the subscriber station are unable to receive all the data in the time division duplex frame.
If the base station and subscriber station were collocated and could instantaneously switch from transmit-to-receive and receive-to-transmit mode, no gaps in transmission would be required for time division duplex. However, the propagation time of the speed of light must be accounted for between separated base stations and subscriber stations.
In addition, base station and subscriber station equipment must be allowed time to switch from a transmitting mode to a receiving mode and vice versa. The timing gaps for time division duplex systems, such as WiMAX systems, are therefore used to account for the propagation time of the signal between the base station and the subscriber station as well as equipment switching times.
The WiMAX Forum release 1.0 approved a profile for a time division duplex frame with a single profile specified for the turn-around times. The turn-around time (timing gap) between the end of the down link frame and the start of the uplink frame, as measured at the base station, is referred to as the transmit transition gap (TTG) in the WiMAX standard. Likewise, the turn-around time between the end of the up link frame and the beginning of the downlink frame, as measured at the base station, is referred to as the receive transition gap (RTG) in the WiMAX standard.
In a distributed antenna system, 5 km of fiber would use up most of the delay budget allowed by the WiMAX profile, resulting in an over-the-air coverage radius of less than 1 km. Fibers of greater length would exceed the delay budget, making the system unusable.